With the trend to more comfort and a more effective energy management of electric loads in vehicles, particularly in future vehicle generations, it is necessary to design also the fans provided to distribute the air in the vehicle interior to be energetically optimized.
A simple possibility to control the speed of d.c. motors step-by-step is changing the slope of the speed line by means of one or several resistors connected to each other in parallel or in series. In case of series-connected resistors, each desired speed requires a separate resistor. The number of the required resistors can be reduced using resistors connected in parallel. Interconnection of just three resistors of the d.c. motor in series results in only four combinations, or speeds, respectively. A higher number of resistors connected to each other increase the number of different speeds and hence the comfort, but cannot be implemented for practical reasons due to the high number of components and the circuit cabling needed.
Another possibility of speed control of d.c. motors raises from using an electronic linear controller. Linear voltage is applied to a transistor so that any speed can be set. Apart from the considerable weight, the main disadvantage is that the power not demanded by the load is changed into heat losses, which the control chip must be continuously release to the environment. Such a release of heat losses is always problematic. Thus, this problem is the main failure cause of overheated transistors.
Another strategy to control speeds is the use of pulse width modulation (PWM) in connection with an inductance coil. A pulsed field-effect transistor (FET) switches and thus limits the current flowing through a coil. The pulsed signal functions to switch on and off the current flowing through the FET, which leads to a smaller power loss. In practice small coils are provided to allow frequencies above the human audibility threshold, i.e. of more than 20 kHz, to be used. The disadvantage of this control strategy is that high frequencies must be applied to the transistor. The fast switching on and off the board voltage in combination with a high-current level leads to high EMC-relevant radiation (electromagnetic compatibility). Additional coils and condensers are necessary to avoid this EMC-relevant radiation in order to meet EMC requirements. This leads to higher costs.
From U.S. Pat. No. 4,856,078 a fan motor, preferably for computers, is known which is series-connected to a pulsed transistor and a voltage source. The pulsed signals with constant amplitudes are produced by a so-called 555 timer whose pulse duty factor can be varied dependent on the temperature by a variable resistor. The pulse frequency is specified to be 10 Hz at a pulse duty factor of 30 to 90% to enable the fan starting. Application of this fan motor, however, is limited to a power range of between 5 and 20 W, because higher powers taken lead to higher levels of noise loading during the start of the motor on the one hand, and create too high heat loads on the other hand.
In U.S. Pat. No. 4,588,934 a control method for vehicle fans is disclosed which includes two generators, one with randomly variable signals and the other with triangular signals, and a comparison element, whereby the comparison element generates a pulsed signal which is applied to a subsequent transistor. The basic idea of this invention is that random speeds of the fan motor are created in order to enhance the passengers' comfort a specific disadvantage of the transistor to which several kHz are applied is that the EMC-relevant radiation has a very high level.
Concerning the spurious radiation requirements, motor vehicles have a special status compared to other products, because their components are locally arranged not very distant from each other, which causes mutual disturbances and interference, particularly by the car radio. Therefore, these components are required to give off a very low spurious radiation in view of electromagnetic fields.
It is one objective of this invention to propose a circuit and a method for the speed control of a d.c. motor that improve the electromagnetic compatibility (EMC), particularly concerning vehicles, and demand a smaller number of electric and/or electronic components to meet the function requirements thus reducing the implementation costs.